A fever is any body temperature at or above 38°C (100.4°F). This threshold applies to adults, children, and infants, and it’s the standard used by the CDC, the American Academy of Pediatrics, and most hospitals worldwide. Normal body temperature sits around 37°C, though healthy people can range anywhere from 36.1°C to 37.2°C depending on the time of day, activity level, and how the temperature is measured.
The 38°C Threshold
The number 38°C is the most widely accepted cutoff for fever. At this temperature or above, your body is mounting an immune response, typically to an infection. Below 38°C but above your personal baseline, you might feel warm or slightly off, but this generally falls into what’s sometimes called a low-grade elevation rather than a true fever.
That said, the familiar “normal” of 37°C is more of an average than a rule. Your body temperature naturally dips in the early morning and peaks in the late afternoon, sometimes varying by half a degree or more. Exercise, heavy clothing, hot weather, and the menstrual cycle can all nudge your temperature up without any illness involved.
Where You Measure Matters
The 38°C threshold assumes a rectal, ear, or forehead (temporal artery) reading. If you’re using a different method, the numbers shift slightly:
- Rectal, ear, or forehead: 38°C or higher is a fever.
- Oral (under the tongue): 37.8°C or higher is a fever.
- Armpit (axillary): 37.2°C or higher is a fever.
Armpit readings tend to run the lowest because the thermometer isn’t inside a body cavity. Rectal readings are the most accurate, which is why they’re the standard for infants. If you’re checking your own temperature orally, keep in mind that drinking something hot or cold within the previous 15 minutes can throw off the result.
Fever Ranges and Severity
Not all fevers carry the same weight. A temperature of 38.3°C feels different from 40°C, and the higher the number climbs, the more attention it deserves.
Between 38°C and 38.9°C, most adults feel achy and fatigued but can manage at home with rest and fluids. Once the temperature reaches 39.4°C (103°F) or higher, adults will typically look and feel noticeably sick, with chills, sweating, headache, and muscle pain becoming more pronounced. At the extreme end, a body temperature above 41.5°C is classified as hyperpyrexia, a rare and dangerous condition that can affect organ function and requires emergency treatment.
Fever Thresholds for Babies and Children
The 38°C cutoff applies to children too, but the urgency depends heavily on age. For any baby younger than 3 months, a rectal temperature of 38°C or higher warrants immediate medical evaluation, even if the baby appears well. Young infants have immature immune systems, and a fever at that age can signal a serious infection that isn’t obvious from the outside.
For infants between 8 and 60 days old, the American Academy of Pediatrics has specific clinical guidelines for managing fevers at or above 38°C, reflecting how carefully this age group needs to be assessed. Older children and toddlers with fevers in the 38°C to 39°C range can often be monitored at home, with the focus on how the child is behaving (drinking fluids, staying alert, responding normally) rather than the exact number on the thermometer.
Quick Celsius-to-Fahrenheit Reference
If you’re used to Fahrenheit and trying to make sense of a Celsius reading, here are the key benchmarks:
- 36.1°C to 37.2°C: Normal range (97°F to 99°F)
- 37.8°C: Fever by oral measurement (100°F)
- 38°C: Fever by rectal, ear, or forehead measurement (100.4°F)
- 39.4°C: High fever in adults (103°F)
- 41.5°C: Hyperpyrexia, a medical emergency (106.7°F)
To convert any Celsius reading to Fahrenheit yourself, multiply by 1.8 and add 32. So 38°C × 1.8 = 68.4, plus 32 = 100.4°F.
Why Your Body Runs a Fever
A fever isn’t a malfunction. It’s your immune system deliberately raising your internal thermostat to create an environment that’s harder for bacteria and viruses to thrive in. Infections are the most common trigger, but fevers can also result from inflammatory conditions, certain medications, heat exhaustion, and, rarely, some cancers.
Because fever is a defensive response, treating it is really about comfort. Bringing your temperature down with a fever reducer doesn’t fight the underlying illness, but it can help you rest, eat, and drink when you otherwise wouldn’t. For most healthy adults, a fever below 39°C that lasts a day or two is uncomfortable but not dangerous on its own.